The present invention relates to a vapor deposition material for coating, which is excellent in heat resistance and thermal shock resistance and capable of forming a stabilized melt pool so that vapor deposition can be carried out.
Thermal spraying has conventionally been a general method for forming a thermal barrier coating film. However, as a result of a desire for performance improvements in thermal barrier coating films, methods for forming a thermal barrier coating film by physical vapor deposition (hereinafter referred to as PVD methods) have been put to industrial use. Among these, an EB-PVD method using an electron beam has attracted attention, and methods for forming a thermal barrier coating film by the EB-PVD method have been developed, such as that described in ADVANCED MATERIAL and PROCESS, Vol. 140, No. 6, No. 12, pp. 18-22(1991).
Such methods for forming a thermal barrier coating film by the EB-PVD method are used mainly for the heat-resistant coating of parts of, e.g. , an aircraft engine. Materials used as this coating material are required to have excellent heat resistance because they are deposited on those areas of parts which come into direct contact with a high-temperature combustion gas. These materials are hence required to have a high melting point and a high purity. Furthermore, these coating materials are desired to have good adhesion to metal parts, not to peel off by heat cycles, not to be corroded by combustion gas components, and to have a low thermal conductivity so as to keep the temperature of the metal parts as low as possible and to thereby enhance the durability thereof; this durability enhancement is the purpose of this coating film. At present, zirconia containing a stabilizer such as yttria is mainly used as a material satisfying these requirements.
This vapor deposition material consisting of stabilizer-containing zirconia, which is used for forming a thermal barrier coating film is subjected to film deposition by the EB-PVD method employing an electron beam capable of evaporating the vapor deposition material with a high energy, since the material has a high melting point and it is necessary to form a vapor deposition film at a high rate.
In this EB-PVD method, the vapor deposition material set in a crucible is abruptly irradiated with an electron beam with a high energy. Upon this EB irradiation, the vapor deposition material breaks due to the thermal shock as long as it is not a molded body having regulated properties. In case where such breakage occurs in the vapor deposition material, there will be a trouble in supplying the vapor deposition material. Consequently, vapor deposition materials which are broken by the electron beam cannot be practically used.
A means for solving the problem of this breakage caused by an electron beam shock has been disclosed which comprise improving the thermal shock resistance of a vapor deposition material by making it porous.
For example, DE 4,302,167 Cl discloses a zirconia sintered body which contains from 0.5 to 25 wt % Y2O3 and has a density of 3.0 to 4.5 g/cm3 and in which the content of monoclinic crystals is from 5 to 80% and the rest is accounted for by tetragonal or cubic crystal. In the case where zirconia which has not been stabilized is used in producing the sintered body, it undergoes a phase change from tetragonal crystal to monoclinic crystal during the temperature drop in a heat treatment for the vapor deposition material production although it is tetragonal crystal and stable at high temperatures. This phase change is accompanied by microcracking. There is a description in that reference to the effects that the resultant sintered body has improved cracking resistance due to the presence of the microcracks resulting from the presence of those microclinic crystals generated by the heat treatment, and that cracking resistance is improved also by using larger particles which have an average particle diameter of 50 xcexcm or smaller and in which the content of 0.4 xcexcm and larger particles is 90% or higher and that of 1 xcexcm and larger particles is 50% or higher.
However, even when the zirconia sintered body which is inhibited from cracking due to the presence of the microcracks generated by the action of monoclinic crystals or which has improved cracking resistance due to the use of larger particles is used, there are cases where sintering proceeds rapidly upon rapid heating during EB irradiation depending on the mixed or solid-solution state of Y2O3, serving as a sintering aid, and the zirconia particles and on, the pore size, whereby the sintered body comes not to withstand the stress accompanying the sintering shrinkage and breaks.
JP-A-7-82019 (the terms xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes a vapor deposition material for heat-resistant coating which comprises a zirconia-based porous sintered body. This sintered body is produced by mixing zirconia particles having a purity of 99.8% or higher and a particle diameter of from 0.1 to 10 xcexcm with yttria particles having a particle diameter of 1 xcexcm or smaller, granulating the mixed powder into spherical granules at least 70 wt % of which have a particle diameter of from 45 to 300 xcexcm, heat-treating the mixture granules to form zirconia granules which are spherical aggregate particles at least 50% of which have a particle diameter of from 45 to 300 m, and sintering the zirconia granules. This sintered body has a porosity of from 25 to 50%, and the pores having a diameter of from 0.1 to 5.0 xcexcm account for at least 70% of all the pores thereof.
This sintered body has better resistance to cracking caused by EB irradiation than conventional vapor deposition materials. However, since this zirconia sintered body has been produced from large particles having a wide particle size distribution and poor uniformity in shape, the vapor deposition material has an uneven microstructure. This vapor deposition material therefore has problems that it is difficult to form a stabilized melt pool when the vapor deposition material is melted by EB irradiation in conducting vapor deposition, and that increasing the EB output so as to heighten the deposition rate tends to result in melt scattering, etc.
As described above, the known techniques have been insufficient in forming a stabilized melt pool and obtaining a stable evaporation rate by improving thermal shock resistance during EB irradiation and preventing the melt which has been melted by EB irradiation from bumping.
An object of the invention is to provide a novel vapor deposition material for heat-resistant coating which has been improved in the formation of a stabilized melt pool and stability of evaporation rate by mitigating or eliminating the problems of those conventional vapor deposition materials, i.e., by improving thermal shock resistance during EB irradiation and preventing the melt which has been melted by EB irradiation from bumping. Another object of the invention is to provide a vapor deposition method wherein the vapor deposition material is used.
The present inventors made intensive studies in order to solve such problems. As a result, the invention has been achieved.
The invention provides a novel vapor deposition material for heat-resistant coating which comprises a zirconia sintered body containing a stabilizer, wherein the sintered body has a content of monoclinic crystals of from 25 to 90% and has a maximum thermal expansion ratio not exceeding 6xc3x9710xe2x88x923 based on room temperature when heated in the temperature range of from room temperature to 1,200xc2x0 C., and which has been improved in the formation of a stabilized melt pool and stability of evaporation rate by improving thermal shock resistance during EB irradiation and preventing the melt which has been melted by EB irradiation from bumping. The invention further provides a vapor deposition method wherein this vapor deposition material is used.
The invention will be described in detail. The vapor deposition material of the invention comprises a zirconia sintered body containing a stabilizer, and is characterized in that the sintered body has a content of monoclinic crystals of from 25 to 90% and has a maximum thermal expansion ratio not exceeding 6xc3x9710xe2x88x923 based on room temperature when heated in the temperature range of from room temperature to 1,200xc2x0 C. This vapor deposition material for. heat-resistant coating is preferably characterized in that the zirconia sintered body has a tapped density of from 3.0 to 5.5 g/cm3, a porosity of from 5 to 50%, and a mode size of pores of from 0.3 to 10 xcexcm and in the sintered body the volume of pores of from 0.1 to 10 xcexcm accounts for at least 90% of the total pore volume.